The present invention relates to a printer and more particularly to a stencil printer for printing an image on a paper or similar recording medium by causing ink to ooze out via perforations formed in a master.
A digital thermal printer using a stencil is conventional and includes a rotatable ink drum. The ink drum is made up of a porous cylindrical base and one or more mesh screens wrapped around the base one above the other and formed of resin or metal. The stencil has a laminate structure consisting of a thermoplastic resin film (generally about 1 .mu.m to 3 .mu.m thick) and a porous support implemented by Japanese paper fibers or synthetic fibers or a mixture thereof. A thermal head selectively perforates the film surface of the stencil by heat in accordance with image data. After the perforated part of the stencil, i.e., a master has been wrapped around the ink drum, ink feeding means arranged in the ink drum feeds ink to the inner periphery of the ink drum. Then, a press roller or similar pressing means presses a paper against the ink drum. As a result, the ink oozes out via the porous portion of the ink drum and the perforations of the master, forming an image on the paper.
In the above conventional stencil printer, after a printing operation using a given master, the stencil is perforated in accordance with the next document in order to produce a new master. The new master is automatically wrapped around the ink drum for effecting the next printing. So long as printing is continuously effected with consecutive masters, a sufficient amount of ink is held between the base and the mesh screen of the ink drum and can be surely fed even to a new master, rendering even the first printing attractive.
On the other hand, assume that a new master is wrapped around the ink drum after the printer has been left unused over a long period of time. Then, the ink existing between the base and the mesh screen of the ink drum is short of water due to evaporation and small in volume. As a result, a substantial period of time is necessary for such ink to infiltrate into the porous support of the new master and ooze out via the perforations of the thermoplastic resin film of the master.
The above ink short of water is low in viscosity and sticky. Should such ink be transferred to a paper via the perforations of the thermoplastic resin film, it would blur an image or would be transferred to the rear of another paper (so-called offset). Particularly, in a duplex print mode for printing images on both sides of a paper, a desirable printing is not achievable until the undesirable ink has been fully consumed. Consequently, several papers to several tens of papers should be wasted before ink capable of forming a desirable image with a new master is fed to the ink drum.
The mesh screen layer is exposed to air and has a great area. The ink deposited on the mesh screen layer and the inner periphery of the ink drum, among others, noticeably decreases in viscosity when the printer is left unused over a long period of time.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 6-40139, 6-71996, 6-135111, 7-257005 and 10-95156 as well as in U.S. Pat. No. 5,782,178.